Wavelength division multiplexed (WDM) optical communication systems (referred to as “WDM systems”) are systems in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber using an optical multiplexer circuit (referred to as a “multiplexer”). Such systems may include a transmitter circuit, such as a transmitter (Tx) photonic integrate circuit (PIC) having a transmitter component that includes a laser associated with each wavelength, a modulator configured to modulate the output of the laser, and a multiplexer to combine each of the modulated outputs (e.g., to form a combined output or WDM signal). Dual-polarization (DP) (also known as polarization multiplex (PM)) is sometimes used in coherent optical modems. A Tx PIC may include a polarization beam combiner (PBC) to combine two optical signals into a composite DP signal.
A WDM system may also include a receiver circuit having a receiver (Rx) PIC. The Rx PIC may include a polarization beam splitter (PBS) to receive an optical signal (e.g., a WDM signal), split the received optical signal, and provide two optical signals (e.g., associated with orthogonal polarizations) associated with the received optical signal. The Rx PIC may also include an optical demultiplexer circuit (referred to as a “demultiplexer”) configured to receive the optical signals provided by the PBS and demultiplex each one of the optical signals into individual optical signals. Additionally, the receiver circuit may include receiver components to convert the individual optical signals into electrical signals, and output the data carried by those electrical signals.
The transmitter (Tx) and receiver (Rx) PICs, in an optical communication system, may support communications over a number of wavelength channels. For example, a pair of Tx/Rx PICs may support ten channels, each spaced by, for example, 200 GHz. The set of channels supported by the Tx and Rx PICs can be referred to as the channel grid for the PICs. Channel grids for Tx/Rx PICs may be aligned to standardized frequencies, such as those published by the Telecommunication Standardization Sector (ITU-T). The set of channels supported by the Tx and Rx PICs may be referred to as the ITU frequency grid for the Tx/Rx PICs.
In a WDM system, the transmitter circuit may modulate a phase of a signal in order to convey data (via the signal) to the receiver circuit where the signal may be demodulated such that the data, included in the signal, may be recovered (e.g., based on the modulated phase). A transmitter circuit may include a digital signal processor (DSP) to apply pulse shaping to the signal to shape the spectrum of the signal (e.g., to minimize inter-symbol interference (ISI) and adjacent channel interference (ACI)), via a roll-off factor of Nyquist filtering. The roll-off factor can be selected to provide a trade-off between spectral efficiency and simplicity in implementation. For example, the roll-off factor may be proportional to simplicity, but inversely proportional to spectral efficiency (e.g., the amount of capacity for a given bandwidth). The roll-off factor can generally be selected in the range of approximately 0.3 to approximately 1 for dense WDM systems.